IIRC Yarns can output pitch CV as low as -3V. You’d have to redesign some parts of the circuit but technically you could modify it to work on +12V alone if you don’t need any pitch CV below 0V.

One benefit of having a negative supply is that your typical CV range would be centered around 0V. That means, your oscillators pitch won’t change much when you plug in the CV signal from yarns.
If you change the CV ranges to something like 0…8V, that would center your range around 4V, meaning that your oscillators will go up by 3 or 4 octaves whenever you plug in the CV.